The rapidity of modern-day data processing imposes severe demands on the ability to produce a printout record at very high speed. Printing systems in which permanently shaped character elements physically contact a recording medium are proving to be too slow and too bulky for many applications. Thus, the industry has turned to other alternatives involving non-impact printing schemes using various techniques to cause a desired character to be formed on the recording medium. Some of these involve the use of electrostatic or magnetic fields to control the deposition of a visible character-forming substance, either solid (i.e., dry powder) or liquid (i.e., ink) on the medium which is usually paper. Other systems utilize electrophotographic or ionic systems in which an electron or ion beam impinges on the medium and causes a change in coloration at the point of impingement. Still another system employs a thermal image to achieve the desired shape coloration change. Of more recent import is a printing technique, called ink jet printing, in which tiny droplets of ink are electronically caused to impinge on a recording medium to form any selected character at any location at very high speed. Ink jet printing is a non-contact system which, in some implementations, requires no specially treated recording media, ordinary plain paper being suitable, and which requires no vacuum equipment or bulky mechanical mechanisms. The present invention relates to this kind of printing system.
The ink jet system to which the invention relates is called an impulse, or ink-on-demand printer, being one in which ink droplets are impelled on demand from a nozzle by thermal energy. The invention is concerned with a nozzle head for this latter type of system.
In a co-pending application, Ser. No. 415,290 filed Sept. 7, 1982 now U.S. Pat. No. 4,490,728 and entitled THERMAL INK JET PRINTER by John L. Vaught et al., and assigned to the instant assignee, an ink-on-demand printing system is described which utilizes an ink-containing capillary having an orifice from which ink is ejected. Located closely adjacent to this orifice is an ink-heating element which may be a resistor located either within or adjacent to the capillary. Upon the application of a suitable current to the resistor, it is rapidly heated. A significant amount of thermal energy is transferred to the ink resulting in vaporization of a small portion of the ink adjacent the orifice and producing a bubble in the capillary. The formation of this bubble in turn creates a pressure wave which propels a single ink droplet from the orifice onto a nearby writing surface or recording medium. By properly selecting the location of the ink-heating element with respect to the orifice and with careful control of the energy transfer from the heating element to the ink, the ink bubble will quickly collapse on or near the ink-heating element before any vapor escapes from the orifice.
It will be appreciated that the lifetime of such thermal ink jet printers is dependent, among other things, upon conductor and resistor lifetime. It has been found that a significant factor in conductor and resistor failure is cavitation damage which occurs during bubble collapse as well as by chemical attack by the ink itself. Hence, it is desirable that resistor wear due to chemical attack and cavitation damage should be minimized as much as possible. In co-pending application Ser. No. 449,820 entitled THERMAL INK JET PRINTER UTILIZING SECONDARY INK VAPORIZTION, filed on Dec. 15, 1982 by John D. Meyer and assigned to the instant assignee, a solution to reducing resistor wear is described. The resistive layer is covered with a passivation layer to provide chemical and mechanical protection during operation. The passivation layer in this application may be a thin layer of such materials as silicon carbide, silicon oxide, or aluminum oxide. In co-pending application Ser. No. 443,972 entitled MONOLITHIC INK JET ORIFICE PLANT/RESISTOR COMBINATION filed Nov. 23, 1982 by Frank L. Cloutier, et al., and assigned to the instant assignee, it is suggested that the passivating or protective layer may be formed initially on the orifice plate of such materials as silicon oxynitride, aluminum oxide or titanium dioxide as well as silicon dioxide. Resistors and conductors are then deposited on this passivation layer. In co-pending application, Ser. No. 444,412 entitled INVERSE PROCESSED RESISTANCE HEATER, filed Nov. 24, 1983, by William J. Lloyd and assigned to the instant assignee, a similar passivation layer of silicon dioxide or silicon carbide is deposited over already-formed resistors and conductors of tantalum/aluminum alloy and aluminum, respectively.
In co-pending application of Friedrich Scheu, Ser. No. 497,774 entitled THERMAL INK JET PRINTHEAD filed May 25, 1983 and assigned to the instant assignee, a passivation structure comprising two distinct layers is disclosed. The upper layer, the one in contact with the ink and on which the ink bubble collapses, is silicon carbide. The underlying layer which covers the resistor structure (phosphorus-diffused silicon) is silicon nitride or oxynitride. The nitride is employed because of its excellent adherence to the materials constituting the resistor structure and the electrical conductors therefor.
While the foregoing passivation materials and techniques have been satisfactory as far as their wear properties are concerned, they are not as free from defects such as pinholes and the like as may be desired. Furthermore, the various structures and layers of the prior art are formed by decomposition-deposition processes, such as plasma enhanced chemical vapor deposition which are expensive to operate. Freedom from defects and pinholes is particularly critical in the case of the layer in contact with the fluid ink to which heat is being transferred from the underlying resistor structure. Irregularities in the surface of this layer, such as may be in the form of partial voids, depressions, or pinholes, may compromise the protection of the underlying layers and/or may result in a non-uniform transfer of heat to the fluid ink volume making it difficult to obtain uniformly-sized bubbles being emitted from the ink jet head at uniform velocities and trajectories.